(1) drug factors ① the influence of dosage, dosage form and route of administration: the dosage is too small and the curative effect is poor; If the dose is too large, it will lead to poisoning reaction or animal poisoning death. The absorption of aqueous solution and injection is faster than that of oil, suspension and solid preparation. The order of action of different routes of administration is: intravenous administration > inhalation administration > intramuscular injection administration > subcutaneous injection administration > oral administration > rectal administration > transdermal administration. ② Time and frequency of administration A. Administration on an empty stomach before meals: fast and sufficient absorption, fast and good curative effect.
B. Administration after feeding: In order to avoid direct damage to gastrointestinal mucosa, it is suitable for drugs with strong irritation.
C. dosage: it depends on the condition and the half-life of the drug. ③ Repeated medication A. Tolerance: After continuous medication, the curative effect will gradually decrease, and it is necessary to increase the dosage to achieve the original effect.
B drug resistance: refers to the decrease of sensitivity of pathogens, parasites and tumor cells to drugs, and the decrease or ineffectiveness of drug efficacy. ④ Drug interactions such as tetracycline and enrofloxacin. It can be complexed with metal ions such as calcium, iron and magnesium in digestive tract, affecting drug absorption or inactivating drugs.
(2) Animal factors ① Physiological factors The response of pigs to the same drug varies with age, sex, pregnancy or lactation, and is closely related to the functional state of organs and tissues, especially to the drug metabolism enzyme system in the liver. For example, the liver detoxification function and kidney detoxification function of newborn piglets are weak. Therefore, the half-life of drugs eliminated by liver microsomal enzyme metabolism and renal excretion in young piglets will be prolonged. Older pigs also have the above phenomenon (decreased liver and kidney function), and are generally more sensitive to drugs than adult animals, so the clinical dosage should be reduced appropriately. ② The functional state of the body is different (pathological state).
Many drugs play an important role in sick animals, even in pathological conditions. For example, antipyretic and analgesic drugs can cool sick pigs, but have no effect on normal pig body temperature. Digitalis has a good cardiotonic effect on chronic congestive heart failure, but it has no obvious effect on normal heart.
Severe liver and kidney dysfunction in pigs can affect the biotransformation and excretion of drugs, cause the accumulation of drugs, prolong the half-life, thus enhancing the role of drugs, and even lead to toxic reactions in severe cases. However, there are also a few drugs that do not take effect until liver biotransformation, such as cortisone and prednisone, and their effects in sick pigs with liver insufficiency are weakened.
In pigs with severe parasitic diseases, hemorrhagic diseases or malnutrition, the plasma protein is greatly reduced, which can increase the blood free drug concentration of drugs with high plasma protein binding rate, which can not only enhance the drug action, but also increase the biotransformation and excretion of drugs and shorten the half-life. (3) Individual difference means that the blood concentration, action and duration of action of the same dose of drug in different individuals are different. Can be divided into high sensitivity and tolerance. Under the same basic conditions, a few individuals of the same animal are particularly sensitive to drugs, which is called high sensitivity; A few individuals are particularly insensitive, which is called tolerance. Individual differences are not only manifested as differences in drug dosage, but also as qualitative differences, that is, individual animals have allergic reactions after taking drugs.
(3) Environmental factors, including poor feeding management, moldy feed, changes in the external environment, seasonal changes, etc., can all affect the drug action.